c++: don't wrap cleanups that can't throw

Since C++11, the vast majority of destructors are noexcept, so
wrap_temporary_cleanups adds a bunch of useless TRY_CATCH_EXPR to be removed
later in the optimizers.  It's simple to avoid adding them in the first
place.

gcc/cp/ChangeLog:

	* decl.c (wrap_cleanups_r): Don't wrap if noexcept.

gcc/testsuite/ChangeLog:

	* g++.dg/eh/cleanup6.C: New test.

gcc/cp/decl.c

@@ -7428,12 +7428,14 @@ wrap_cleanups_r (tree *stmt_p, int *walk_subtrees, void *data)
       tree guard = (tree)data;
       tree tcleanup = TARGET_EXPR_CLEANUP (*stmt_p);
-      tcleanup = build2 (TRY_CATCH_EXPR, void_type_node, tcleanup, guard);
-      /* Tell honor_protect_cleanup_actions to handle this as a separate
-	 cleanup.  */
-      TRY_CATCH_IS_CLEANUP (tcleanup) = 1;
- 
-      TARGET_EXPR_CLEANUP (*stmt_p) = tcleanup;
+      if (tcleanup && !expr_noexcept_p (tcleanup, tf_none))
+	{
+	  tcleanup = build2 (TRY_CATCH_EXPR, void_type_node, tcleanup, guard);
+	  /* Tell honor_protect_cleanup_actions to handle this as a separate
+	     cleanup.  */
+	  TRY_CATCH_IS_CLEANUP (tcleanup) = 1;
+	  TARGET_EXPR_CLEANUP (*stmt_p) = tcleanup;
+	}
     }
   return NULL_TREE;

gcc/testsuite/g++.dg/eh/cleanup6.C

+// Test that we don't wrap the non-throwing A cleanup with a B cleanup.
+
+// { dg-do compile { target c++11 } }
+// { dg-additional-options -fdump-tree-gimple }
+// { dg-final { scan-tree-dump-times "B::~B" 1 "gimple" } }
+
+struct A { A(); ~A(); };
+struct B { B(const A& = A()); ~B(); };
+
+int main()
+{
+  B b;
+}